Field of the Invention
The present invention relates to an optical fiber characteristic measuring device, and particularly to an optical fiber characteristic measuring device which measures characteristics of an optical fiber on the basis of backscattered light generated due to Brillouin scattering in the optical fiber to be measured.
Priority is claimed on Japanese Patent Application No. 2015-175901, filed on Sep. 7, 2015, the contents of which are incorporated herein by reference.
Description of Related Art
Brillouin scattering generated when light is incident on optical fibers, which are one of optical transmission media, is changed in accordance with a strain applied to the optical fibers or a temperature of the optical fibers. A method of measuring a strain distribution or a temperature distribution in a longitudinal direction of optical fibers by measuring an amount of frequency shift of light caused by Brillouin scattering is known. For example, a strain generated on structures such as bridges and buildings can be detected by stretching optical fibers around the structures, and specifying distorted places of the optical fibers on the basis of the above-described method. So-called Brillouin optical time domain reflectometry (BOTDR) and Brillouin optical correlation domain reflectometry (BOCDR) methods are known as such a measuring method.
As disclosed in Japanese Patent No. 3095694, a measuring method of a BOTDR method includes detecting Brillouin scattered light obtained when a light pulse is incident on one end of an optical fiber to be measured, and measuring an amount of frequency shift of the Brillouin scattered light with respect to the incident light (hereinafter referred to as an amount of Brillouin frequency shift) and a time until the Brillouin scattered light is returned. The Brillouin scattered light is backscattered light scattered due to an acoustic wave whose speed is changed depending on a strain or a temperature of the optical fiber to be measured. A magnitude of the strain or a temperature of the optical fiber to be measured can be measured by measuring the above-described amount of Brillouin frequency shift, and a position in a longitudinal direction of the optical fiber to be measured can be specified by measuring a time until the Brillouin scattered light is returned.
There is a need to narrow a pulse width of an optical pulse incident on one end of the optical fiber to be measured to improve a spatial resolution in the above-described measuring method of the BOTDR method. However, when a pulse width of an optical pulse is narrow, a signal intensity of backscattered light generated in an optical fiber to be measured is reduced, and a signal-to-noise ratio (hereinafter referred to as an SNR) is deteriorated in some cases. In some cases, it takes a lot of times for measurement because it is necessary to increase a process of integrating backscattered light to improve the deteriorated SNR.
On the other hand, a measuring method of a BOCDR method includes detecting Brillouin scattered light obtained when a pump light serving as a frequency modulated continuous wave of light is incident on one end of an optical fiber to be measured, and measuring an amount of Brillouin frequency shift. As disclosed in Japanese Patent No. 5105302 and Yosuke MIZUNO, Zuyuan H E, and Kazuo NOTATE, “Stable Entire-length Measurement of Fiber Strain Distribution by Brillouin Optical Correlation-domain Reflectometry with Polarization Scrambling,” The Institute of Electronics, Information and Communication Engineers, the 2009 IEICE General Conference, C-3-88, Ehime University, Mar. 17 to 20, 2009, in the measuring method of the BOCDR method, Brillouin scattered light at a specific position, which is called a correlation peak, in an optical fiber to be measured is selectively extracted by making the Brillouin scattered light and reference light to interfere with each other. For example, when a continuous wave of light which undergoes sinusoidal wave frequency modulation is incident on an optical fiber to be measured, an interval of correlation peaks in the optical fiber to be measured is inversely proportional to a modulation frequency of the sinusoidal wave frequency modulation. For this reason, a modulation frequency of the continuous wave of light is adjusted such that only one correlation peak is in the optical fiber to be measured, and thereby only scattered light generated at a position associated with the correlation peak can be extracted, and a correlation peak can be moved in a longitudinal direction of the optical fiber to be measured by sweeping the modulation frequency of the continuous wave of light. A strain distribution or a temperature distribution in the longitudinal direction of the optical fiber to be measured can be measured by acquiring an amount of Brillouin frequency shift at correlation peaks while moving the correlation peaks.
In the above-described measuring method of the BOCDR method, since Brillouin scattered light in a narrow region of about several cm in an optical fiber to be measured can be selectively output as an interfering output associated with a specific position in a longitudinal direction of the optical fiber to be measured, the measuring method of the BOCDR can realize a spatial resolution which is two orders higher than that of the above-described measuring method of the BOTDR method. Also, since a continuous wave of light is incident on an optical fiber to be measured rather than an optical pulse, a signal intensity of backscattered light generated in the optical fiber to be measured is high, and thus it is easy to measure the signal intensity. Since a process of integrating backscattered light as in the measuring method of the BOTDR method is not needed, a measurement time can be reduced.
In a measuring method of a BOCDR method, since backscattered light generated in an optical fiber to be measured and reference light output by a light source are caused to interfere with each other, a signal intensity after the interference depends on polarization states of both of the lights. In the related art, polarization dependence has been reduced by providing a polarization scrambler (PSCR) which rotates a polarization plane at high speed on one of an optical path of reference light, an optical path of backscattered light, and an optical path of pump light to acquire a stable interference signal.
A PSCR is a device which averages influences of polarization states by changing the polarization states at high speed (at about a frequency of MHz). For example, an observed Brillouin gain spectrum (BGS) does not depend on a relative polarization state between reference light and backscattered light by providing a PSCR on an optical path of the reference light. The polarization states can be averaged by increasing an operation speed of the PSCR to a speed which is several tens of times faster than a sampling speed of backscattered light. However, since there is a limit to an operation speed of the PSCR, when the sampling speed of backscattered light is increased, an effect of the averaging is decreased, and unnecessary fluctuation depending on the polarization state thus appears in measurement data.
A method of eliminating an effect of polarization by performing measurement on an arbitrary polarization plane, performing measurement on a polarization plane which is rotated 90° with respect to the polarization plane, and acquiring the square root of a squared sum of the measured values, that is, acquiring a vector sum, is known as another method for reducing polarization dependence. The same effects as a PSCR can be acquired using this method. However, since there is a need to perform measurement twice by switching polarization planes, this method has a measurement time which is two times longer than when measurement is performed once. When a change of a polarization plane is slow and a sampling speed of backscattered light is slow, polarization states are substantially the same even if measurement is performed twice while switching polarization planes. Thus, the effect of polarization can be eliminated by acquiring the square root of the squared sum of the two measured values. However, when an optical fiber vibrates at a high speed or the like and the polarization plane is likely to rapidly rotate, the polarization states are changed between the two measurements. Thus, the effects of polarization cannot be eliminated even if the square root of the squared sum of the two measured values is acquired.